Most of the offshore drilling rigs used in the oil and gas producing industry are of the jack-up type. A jack-up type off-shore drilling rig generally comprises a floatable barge having three or more legs vertically disposed through said barge which may be lowered to the bottom of a body of water for supporting said barge above the surface of said body of water. The offshore drilling rig is elevated or "jacked-up" on its supporting legs during oil and gas producing operations.
An offshore drilling rig typically spends approximately ninety percent to ninety-five percent of its working life in an elevated position. The remainder of the time is spent in transit moving from one drilling site to another. Typically, offshore drilling rigs are not self propelled and must be towed to and from drilling sites by large tugs. Normally, the time it takes to tow an offshore drilling rig from one site to another is not productive time for the owners of the offshore drilling rig. Because an offshore drilling rig generally does not generate revenue during the time it is in transit, it is desirable to make the transit time as short as possible. Therefore, any increase which may be achieved in the speed at which an offshore drilling rig may be towed is highly desirable.
The towing speed of an offshore drilling rig may be increased by decreasing the towing resistance of the barge hull of the offshore drilling rig. Because the towing resistance of any particular barge hull is related to the shape of that barge hull, it is possible to reduce the towing resistance of a particular type of barge hull by making changes in the barge hull's structural design. Unfortunately, many of the structural designs that would reduce towing resistance in a barge hull are either impractical or far too expensive in construction.
The present invention is directed toward providing an inexpensive means for reducing towing resistance in an offshore drilling rig barge hull. A major portion of the towing resistance in a barge hull is due to what is referred to as "form drag." Form drag refers to the frictional forces acting on the towed structure that are specifically attributable to the shape or form of the towed structure. The shape of an offshore drilling rig barge hull is usually either triangular or rectangular. In the case of either a triangularly shaped barge hull or a rectangularly shaped barge hull the rear edge of said barge hull as said barge hull is being towed is generally perpendicular to the direction in which said barge hull is being towed. Said rear edge is generally formed having a vertically disposed flat surface extending laterally across the width of the rearmost portion of said barge hull and extending vertically from the bottom to the top of said barge hull.
The hydrodynamic forces of the water moving past the ends of the rear edge of a barge hull cause a suction effect that tends to retain a relatively large body of water immediately behind the barge hull as it is being towed. The natural flow lines of fluid around and under such a barge hull do not generate a sufficiently large amount of hydrodynamic turbulence to cause the water immediately behind the barge hull to flow away from the barge hull. In effect, a large amount of water is being towed along with and behind the barge hull.